We gave the entire (well, no-PII, no active secbugs, etc) contents and history of Bugzilla to some very smart PhDs at uMontreal, and one of the things they came back with was this: somebody submitting their first bugfix was relatively unlikely to submit a second, but somebody on their fourth bugfix is quite likely to get to five, and six, and stick around.

I cannot emphasize enough how much you get from up front code ergonomics and human care in the early stages of the contributor relationship.